Fischer-tropsch catalyst

ABSTRACT

The present invention relates to a catalyst comprising particles of a cobalt and zinc co-precipitate, having a volume average particle size of less than 150 μm. Another aspect of the invention is the use of such a catalyst in a Fischer-Tropsch process. 
     The present invention further relates to a method for preparing a catalyst comprising cobalt and zinc oxide, wherein an acidic solution comprising zinc ions and cobalt ions and a alkaline solution are contacted and the precipitate is isolated.

The invention relates to a Fischer-Tropsch catalyst comprising cobaltand zinc, as well as to a method for preparing such a catalyst.

A catalyst containing cobalt oxide and zinc oxide for use in thesynthesis of C1-C3 aliphatic hydrocarbons is known from U.S. Pat. No.4,039,302.

U.S. Pat. No. 4,826,800 describes a process for preparing a catalystcomprising cobalt and zinc oxide for use after reductive activation as acatalyst in the conversion of synthesis gas to hydrocarbons. Thecatalyst is prepared by mixing a solution of a soluble zinc salt and asoluble cobalt salt with a precipitant such as ammonium hydroxide orammonium carbonate and recovering the precipitate. The ratio ofcarbonate to metal is high in the described method, which has been founddetrimental to the strength of the catalyst.

U.S. Pat. No. 5,345,005 relates to a Cu—Zn catalyst on alumina for thepreparation of alcohols by hydrogenation of e.g. a ketone. In acomparative example, the preparation of a Cu—Zn—Co catalyst on aluminais described, wherein use is made of soda ash. However, the use of sodaash is found to be potentially detrimental to the strength of thecatalyst. The particle size distribution range within which 90% of thevolume of the Cu—Zn—Co catalyst described in U.S. Pat. No. 5,345,005lies, is not specified. It is however expected that the use of soda ashin the preparation of the catalyst leads to a broadening in the particlesize distribution.

U.S. Pat. No. 5,945,458 and U.S. Pat. No. 5,811,365 describe aFischer-Tropsch process in the presence of a catalyst composition of agroup VIII metal, e.g. cobalt, on a zinc oxide support. Such a catalystis made by first preparing the support by adding a solution of zinc saltand other constituents to an alkaline bicarbonate solution. Next, theprecipitate is separated from the bicarbonate solution by filtration toform a filter cake, which can thereafter be dried, calcined and loadedwith the group VIII metal. The catalyst material is then formed intotablets, which tablets are crushed to form particles with a size of250-500 μm, that can be used in a Fischer-Tropsch process. Additionalpost-treatments such as crushing, are required in order to obtain acatalyst powder with good strength properties. However, the obtainedaverage particle size, as indicated above, is still relatively large.Moreover, crushing results in a broad particle size distribution andcatalysts with such a large particle size and a broad particle sizedistribution tend to be less suitable for processes involving a bubblecolumn, a slurry phase reactor or a loop reactor.

WO-A-01/38269 describes a three-phase system for carrying out aFischer-Tropsch process wherein a catalyst suspension in a liquid mediumis mixed with gaseous reactants in a high shear mixing zone, after whichthe mixture is discharged in a post mixing zone. Thus mass transfer issaid to be enhanced. As suitable catalysts inter alia cobalt catalystson an inorganic support, such as zinc oxide are mentioned. The surfacearea of the support used for the preparation of these known catalysts isless than 100 g/m². These prior art cobalt based catalysts can beprepared by depositing cobalt on a suitable support, such as a zincoxide support, by impregnation methodology. Other conventionalpreparation methods include precipitation routes, which typicallyinvolve crushing of a hard filter cake of catalyst material, resultingfrom the catalyst preparation process, into small particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a particle with a multi-lobed spherical geometry accordingto the invention.

FIG. 2 shows the equivalent circle of the particle shown in FIG. 1.

It has however been found that these conventional catalysts do notalways satisfy the requirements with respect to mass transfer and/orheat transfer, when used in a catalytic process.

Further it has been found that the dispersion behaviour of theseconventional catalysts—when used in a slurry phase process—is notparticularly good, since the catalyst particles tend to agglomerate.

Other problems with commercially available zinc oxide supports suitablefor loading with cobalt to form a catalyst, include inappropriateparticle size distribution (in particular with supports obtained byprecipitation), low surface area which typically makes them moredifficult to impregnate, thus several impregnation steps are required toobtain a reasonable amount of cobalt loading on the support and a lowlevel of homogeneity of the cobalt distribution, once the cobalt hasbeen applied.

It is an object of the present invention to provide a novel catalyst,suitable for use in Fischer-Tropsch synthesis, that may be used as analternative to known catalysts.

It has been found that certain catalysts comprising cobalt and zincoxide, with a particular particle size and particle size distributionhave very favourable properties as Fischer-Tropsch catalysts.

Accordingly, the present invention relates to a catalyst comprisingparticles of a cobalt and zinc co-precipitate, having a volume averageparticle size of less than 150 μm and a particle size distributionwherein at least 90% of the volume of the catalyst particles have a sizebetween 0.4 and 2.5 times the average particle size.

The volume average particle size and particle size distribution as usedherein are as determined by a laser diffraction equipment, using aMalvern Master sizer MS 20 apparatus (Program contains 3 minutes, 25%(of maximum power) ultrasonic treatment before particle sizedistribution measurement; Calculation model: Model Independent;Presentation: 1907; see also Examples)

It has been found that a catalyst according to the present invention hasvery favourable properties for use in catalytic processes. A catalystaccording to the invention has been found to have a particular good massand/or heat transfer properties, when used in a catalytic process.

A catalyst according to the invention has been found to be particularlyfavourable for use in a stirred slurry-phase reactor, bubble-columnreactor, loop reactor or fluid-bed reactor.

A catalyst according to the invention shows very good flow properties indry form and/or when used in a stirred slurry reactor, and gooddispersibility properties with the reactants in the reaction mixture.The catalyst of the invention has a very appropriate particle sizedistribution, as indicated by the free-flowing properties of the driedcatalyst, as can be observed, for example, when the catalyst is kept ina storage flask.

A catalyst according to the invention shows very favourable separationproperties and can for example very suitably be separated from thereaction mixture by filtration.

A catalyst according to the invention has an extremely good balancebetween activity and separation properties

Catalysts according to the invention can inter alia be made by theco-precipitation of solutions containing Co- and Zn-precursors. Theobtained co-precipitates (solids) can be post-treated and finallyreduced to yield a Co on zinc oxide catalyst. Very suitable examples ofco-precipitates include co-precipitates of Co/Zn oxide and Co/Zncarbonate, co-precipitates of Co/Zn hydroxide and Co/Znhydroxycarbonate, and combinations thereof.

Preferably the volume average particle size of the catalyst is less than100 μm, more preferably less than 50 μm. The lower limit is notparticular critical. For practical purposes it is preferred that thesize is at least such that the particles can still be separated from aliquid reaction mixture. Particularly suitable is for example a catalystwith an average particle size of 2 μm or more. Very good results havebeen achieved with a catalyst having a volume average particle size inthe range of 5-50 μm.

With respect to the particle size distribution it is preferred that theamount of particles having a size of less than 0.4 times the averageparticle size is much lower (e.g. at least 5 times lower) than theamount of particles having a particle size of more than 2.5 times theaverage particle size. More preferably essentially none of the particlesof the catalyst have a particle size of less than 0.4 times the averageparticle size.

Very good results have been obtained with a catalyst having a particlesize distribution wherein at least 90% of the volume of the catalystparticles have a size between 0.5 and 2.2 times the average particlesize, more preferably between 0.6 and 2 times the average particle size.

Preferably the pore volume of the catalyst—as determined by nitrogenadsorption (N₂-BET), measured on an Ankersmit Quantachrome Autosorb-6apparatus, after degassing the sample at 180° C. to a pressure of 3.3 Pa(25 mTorr)—is at least mainly formed by pores having a diameter in therange of 5-100 nm. Much preferred wherein there are essentially no poreswith a diameter of less than 5 nm (in particular less than 5% of thepore volume formed by pores with a diameter of less than 5 nm). It hasbeen found that such a catalyst has particularly good diffusionproperties for reactant and product. Such a catalyst has also been foundto be highly selective towards the Fischer-Tropsch reaction.

Very good results have been achieved with a catalyst having a porevolume of less than 0.5 ml/g. The pore volume is preferably at least0.05 ml/g. Particularly suitable is a catalyst with an pore volume ofless than 0.45 ml/g.

Such a catalyst has been found to have particularly good physicalstrength properties, which is advantageous in applications in varioustypes of reactors, including slurry-phase reactors, loop-reactors,bubble-column reactors and fluid-bed reactors.

Also the surface area as determined by Ankersmit Quantachrome Autosorb-6apparatus, after degassing at 180° C. down to a pressure of 3.3 Pa (25mTorr), can be chosen within the wide range, depending upon the intendedpurpose. For a Fischer-Tropsch process, this parameter may for examplebe chosen in the range of 1-120 m²/g. Preferably a catalyst has asurface area in the range of 5-100 m²/g Very good results have beenachieved with a catalyst having a surface area in the range of 5-80m²/g.

A preferred catalyst according to the invention is a particulatematerial wherein the particles have a more or less spherical geometry.Such a catalyst has been found to have very good strength and separationproperties, and a relatively high attrition resistance during use.

Very suitable is a more or less spherically shaped catalyst wherein atleast the majority of the particles have a multi-lobed sphericalgeometry. An example of a particle with a multi-lobed spherical geometryis shown in FIG. 1. Particular good results, e.g. with respect to heattransfer and/or mass transfer properties, have been achieved with acatalyst wherein at least the majority of the particles are multi-lobedparticles having a surface area that is at least 1.05 times, preferablyat least 1.1 times, more preferably at least 1.2 times the surface areaof the so called equivalent circle. The term “equivalent circle” is usedherein to describe the largest circumference circle that can just fitwithin the outline of the particle, when the particle is projected (e.g.via a micrograph) onto a plane such that the orientation in viewpresents the maximum possible external surface area that can be seen inany view (see also FIG. 2, for an impression of the equivalent circle ofthe particle shown in FIG. 1).

The composition of the catalyst can be varied widely, which compositionthe skilled professional will know to determine, depending upon theintended purpose.

Preferably, the zinc to cobalt atomic ratio is in the range of 40 to0.1, more preferably in the range of 20 to 0.3.

The catalyst may essentially consist of cobalt and zinc oxide. It ishowever also possible that the catalyst contains one or more othercomponents, such as components that are commonly employed inFischer-Tropsch catalysts. For example the catalyst may contain one ormore promoters, such as ruthenium, hafnium, platinum, zirconium,palladium, rhenium, cerium, lanthanum or a combination thereof. Whenpresent, such promoters are typically used in a cobalt to promoteratomic ratio of up to 10:1.

It has been found that a catalyst according to the invention comprisingat least one group IIIa element, preferably in a concentration of 0.1-10wt % based upon the total weight of the catalyst, has a very favourablestructural stability. Preferred group IIIa elements include aluminium(Al), gallium (Ga) and borium (B), of which aluminium is particularlypreferred.

Very good results have been obtained with a catalyst according to theinvention which is essentially free of sodium. It has been found that acatalyst containing a relatively high amount of sodium is reduced instrength. Further, the presence of sodium has been found to poison thecatalyst, reducing its Fischer-Tropsch activity. Therefore, a catalystwith a sodium content of less than 0.5 wt. %, more in particular of 0 to0.15 wt. %, even more in particular of 0 to 0.1 wt. % based upon theweight of the catalyst, is preferred.

Very good results have been achieved with a catalyst according to theinvention having a low content of copper or being essentially free ofcopper. Copper may stimulate side reactions, such as the formation of analcohol by hydrogenation of a ketone, an aldehyde or a carboxylic acid,which are usually preferably avoided or suppressed, especially in aFischer-Tropsch process. The copper content is preferably less than 2wt. %, more preferably 0 to 0.5 wt % even more preferably 0 to 0.2 wt.%, based upon the weight of the catalyst.

The present invention further relates to a method for preparing acatalyst comprising cobalt and zinc oxide by co-precipitation of cobaltand zinc ions, wherein an acidic solution comprising zinc ions andcobalt ions and an alkaline solution are supplied to a reactorcomprising an aqueous medium, preferably water or an aqueous solution,wherein the acidic solution and alkaline solution are contacted in theaqueous medium and a precipitate comprising cobalt and zinc is formed.The precipitate is thereafter separated from the aqueous medium (whichmay have formed a slurry together with the precipitate). The separatedcobalt and zinc comprising precipitate is then dried and may bepost-treated, e.g. calcined, etc., to form said catalyst.

The combination of acidic solution and alkaline solution is preferablychosen such that the components of the acidic solution and of thealkaline solution are soluble in the aqueous medium, but that the cobaltand zinc precipitate when they are contacted with the alkaline solution,while the counter ions of zinc and cobalt substantially remain insolution. The skilled professional will know how to choose appropriateconditions, such as the type of counter ions and the concentrations foreach of the components.

This method has been found to be particularly suitable for preparing acatalyst as described above.

It has been found that a method according to the invention allows thedirect preparation of a particulate precipitate that acts as a freeflowing catalyst precursor, directly after drying, i.e. it allows thepreparation of a precipitate that does not have to be crushed orotherwise mechanically treated to form a particulate material.

Also a method according to the invention allows the preparation ofparticles with a more or less spherical, optionally multi-lobed,geometry.

Preferably the precipitation of particles is carried out at asubstantially constant pH, in particular at a pH value varying at most±0.2 pH units around a set-point value. Thus it has been found possibleto make a catalyst precursor with very favourable free flowingcharacteristics.

Preferably, the alkaline solution and the acidic solution are suppliedto the reactor simultaneously (from separate conduits).

Optionally the cobalt in the isolated and dried precipitate or calcinedproduct is reduced to metallic cobalt

Suitable sources for ionic zinc respectively ionic cobalt include saltsthereof that are soluble in the acidic solution and in water in asufficient concentration. Preferred examples of such salts include zincnitrate respectively cobalt nitrate and zinc acetate respectively cobaltacetate and other inorganic or organic salts of cobalt respectively zincthat have a similar solubility in the acidic solution

Suitable components for co-precipitating with the cobalt ions and zincions present are inorganic salts and organic salts that are soluble inan aqueous alkaline solution in a sufficient concentration, such ashydroxides, carbonates, urea, isocyanates and any other salt that can beused as base source and that can be dissolved water of in the alkalinesolution. Preferred examples of such salts include ammonium carbonate,ammonium bicarbonate and other inorganic or organic salts of carbonatethat have at least a similar solubility in the alkaline solution.

Preferably, the total concentration of zinc and cobalt ions in theaqueous medium is chosen in the range of 0.1 to 5 moles/litre. Theconcentration is preferably kept within this range throughout theprecipitation step.

The pH of the acid solution is preferably in the range of 1-5. The pH ofthe alkaline solution is preferably in the range of 6-14. The pH in theaqueous medium (wherein the co-precipitation takes place) is preferablyin the range of 4-9, depending upon the type of precursor salts used asa source for cobalt, zinc and alkaline component(s).

The stirring frequency is very suitably chosen to obtain a power inputin the range of 1-300 kW/l aqueous medium. Very good results have beenachieved with a power input in the range of 10-100 kW/l aqueous medium.

The temperature during the co-precipitation process is preferably chosenin the range of 5-98° C., more preferably in the range of 15-75° C.

The present invention further relates to the use of a catalyst accordingto the invention in a slurry reactor, a loop reactor, a bubble-columnreactor or a fluid-bed reactor. The present invention further relates tothe use of a catalyst according to the invention in a Fischer-Tropschprocess or a functional group hydrogenation process, such as nitrilehydrogenation to amines.

The invention is further illustrated by the following examples.

EXAMPLE 1 Catalyst Preparation

A metal solution (1000 ml) containing 10.0 g/l cobalt and 72.3 g/l zincwas prepared by dissolving 329 g of Zn(NO₃)₂.9H₂O and 49.4 g ofCo(NO₃)₂.6H₂O in 1000 ml of demineralised water. The base solution wasprepared by dissolving 154 g of (NH₄)₂CO₃ in 1000 ml of demineralisedwater. The metal and base solution were injected simultaneously at equalflow rates (1000 ml/hr) into a well stirred, baffled precipitationvessel containing 1750 ml of demineralised water. The temperature duringprecipitation was maintained at 60° C. The input power (N) was 0.5 Wattper liter; calculated using the formula

$N = \frac{{kx}\; \rho \; {xn}^{3}{xd}^{5}}{V}$

With:

N=input power of turbine impeller (Watt)

k=factor 6 for a turbine impeller

ρ=stirred liquid density (kg/m³)

n=agitation rotational speed (s⁻¹)

d=agitator diameter (m)

V=volume of precipitation vessel (3.5 litres)

The pH was kept constant at pH 5.8 by providing acid solution andalkaline solution at equal addition rates.

The resulting precipitate was washed with demineralised water and driedovernight at 110° C. The dried catalyst was heated from room temperaturewith 150° C./hr to 500° C. and calcined for 5 hours at 500° C. Theproperties of the calcined catalyst are summarised in Table 1.

EXAMPLE 2 (Comparison) (e.g. as Described in U.S. Pat. No. 4,826,800)

A metal solution containing 20.0 g/l Co and 64.3 g/l Zn was prepared bydissolving 292 g Zn(NO₃)₂.6H₂O and 98.7 g Co(NO₃)₂.6H₂O in 2.6 ldemineralised water to form an acid solution. In the precipitationvessel, a base solution was prepared by dissolving 675 g (NH₄)₂CO₃ in5.2 litre of demineralised water. The acid solution was injected with 12ml/min into the precipitation vessel, containing the base solution,while stirring at room temperature (300 RPM). During the addition, thepH dropped from 9.2 (initial) to 8.4 (final).

The resulting precipitate was washed with demineralised water and driedovernight at 110° C. Some mechanical treatment was required to get apowder out of the dried filtercake. This powder showed no free flowingbehaviour. The dried catalyst powder was calcined at 500° C. for 5 hours(ramp rate 150° C./h).

EXAMPLE 3 Catalyst Characterisation Data and Comparison withConventional Catalyst

Table 1 describes the properties of the catalyst according to theinvention and a comparison with a corresponding conventional catalyst

TABLE 1 Catalyst according Comparison to Example 1 catalyst (Example 2)Cobalt content wt % 7.1 26.3 BET-surface area m²/g 16 41 N₂ pore volumeml/g 0.20 0.55 Particle size distribution D(v. 0.9)¹ μm 34.3 96.2 D(v.0.5)¹ μm 25.3 5.7 D(v. 0.1)¹ μm 19.7 1.5 Span¹ 0.6 (very narrow) 16.5(very broad) Crystallite size² Å 154 143 ad 1: The span is calculatedfrom the measured Malvern particle size distribution and gives anindication for the broadness of the particle size distribution, as isdefined as follows:${Span} = \frac{{D\left\lbrack {v,0.9} \right\rbrack} - {D\left\lbrack {v,0.1} \right\rbrack}}{D\left\lbrack {v,0.5} \right\rbrack}$wherein:D[v,0.9]=particle size (μm) below which 90% of particles exists (inMalvern volume particle size distribution).D[v,0.5]=particle size (μm) below which 50% of particles exists (inMalvern volume particle size distribution).D[v,0.1]=particle size (μm) below which 10% of particles exists (inMalvern volume particle size distribution).ad 2: The CO₃O₄ crystallite size, as reported in table 1, is calculatedfrom the XRD spectrum, particularly from the d=2.03 line in the XRDpattern (CuK□-radiation).

The cobalt content herein was measured by X-ray fluorescence.

EXAMPLE 4 Measurement of the Particle Size Distribution

The particle size distribution of a catalyst according to the inventionwas measured on a Malvern Mastersizer MS 20.

The sample vessel of the apparatus was filled with demineralized water,

and diffraction of measuring-cell filled with water was determined (forbackground correction). An appropriate amount of catalyst powder wasthen added to the sample vessel, which was treated in ultrasonic bathfor 3 minutes (25% of max. output u.s. power) and stirring (50% of max.stirring speed), prior to the measurement. After this treatment, thesample was measured and themeasured diffraction signal was corrected for the ‘background’measurement.

Calculation of particle size distribution was done, using the followingparameters: Model: Model Independent; Presentation: 1907; Particle sizedistribution: Volume distribution).

EXAMPLE 5 Catalytic Performance of Catalyst in Fischer-Tropsch Reaction

A catalyst with a cobalt content of 20 wt. % was prepared. Apart fromthe different cobalt content, the preparation conditions were the sameas in Example 1.

A sample of catalyst (20 g) was reduced in a 3.5 cm OD tubular reactor.The reactor was purged with nitrogen at a space velocity (GHSV) of 1000h-1 at atmospheric pressure. The temperature was raised at 2° C./min to60° C. The gas feed was then switched over to air at 1000 GHSV. Thetemperature was then raised at 1° C./min up to 250° C. and held therefor 3 hours. The gas flow was then changed to nitrogen at 1000 GHSV for6 minutes and then the feed gas was switched to carbon monoxide at 1000GHSV and held for 3.5 hours.

The feed gas was then changed back to nitrogen and the temperatureramped at 4° C./min up to 280° C. Once at 280° C., the feed gas was thenswitched to hydrogen at 2500 GHSV and held there for 10 hours. Thereactor was then cooled to room temperature and purged with nitrogenprior to transfer to the reactor.

The catalyst was transferred under nitrogen purge to a 600 ml continuousstirred tank reactor (CSTR) that had been filled with squalane (300 ml;Aldrich). The reactor was sealed and heated up to 125° C. with anitrogen flow of 250 ml/min. The feed gas to the reactor was thenswitched to syngas at 8000 GHSV, the stirrer speed increased to 700 rpmand the temperature ramped at 2° C./min up to 130° C. The reactor wasthen pressurised to 20 barg at 30 bar/hr. The temperature was thenramped at 60° C./hour up to 160° C., 5° C./hour up to 175, 1° C./hour upto 185, 0.5° C./hour up to 205° C. and 0.3° C./hour up to 212° C.Automatic temperature control was then used to maintain the % COConversion at 60%.

After 40 hours on stream a C5+productivity of 608 g/litre of catalyst/hrwas obtained at a temperature of 226° C.

1. Catalyst comprising particles of a cobalt and zinc co-precipitate,said particles having a volume average particle size of less than 150 μmand a particle size distribution wherein at least 90% of the volume ofthe catalyst particles have a size between 0.4 and 2.5 times the averageparticle size.
 2. Catalyst according to claim 1, wherein the volumeaverage particle size is less than 100 μm, preferably 2 to 50 μm. 3.Catalyst according to claim 1 or 2, wherein the pore volume is mainlyformed by pores having a diameter within the range of 5-100 nm. 4.Catalyst according to any of the preceding claims, wherein the porevolume is less than 0.5 ml/g, preferably less than 0.45 ml/g. 5.Catalyst according to any of the preceding claims, wherein the surfacearea is less than 120 m²/g, preferably in the range of 5-100 m²/g. 6.Catalyst according to any of the preceding claims, wherein the zinc tocobalt atomic ratio is in the range of 40 to 0.1
 7. Catalyst accordingto any of the preceding claims, predominantly comprising particles witha multi-lobed spherical geometry.
 8. Catalyst according to claim 7,wherein the multi-lobed particles have a surface area that is at least1.05 times, preferably at least 1.1 times the surface area of theequivalent circle, wherein the equivalent circle is defined as thelargest circumference circle that can just fit within the outline of theparticle, when the particle is projected onto a plane such that theorientation in view presents the maximum possible external surface areathat can be seen in any view.
 9. Catalyst according to any of thepreceding claims, wherein the copper content is less than 2 wt. % basedupon the total weight of the catalyst, preferably less than 0.5 wt. %based upon the total weight of the catalyst.
 10. Method for preparing acatalyst according to any of the preceding claims, wherein an acidicsolution comprising zinc ions and cobalt ions and an alkaline solutionare supplied to a reactor comprising an aqueous medium, wherein theacidic solution and alkaline solution are contacted in the aqueousmedium, wherein a precipitate comprising cobalt and zinc are formed,after which the precipitate is isolated from the aqueous medium driedand post treated to form said catalyst.
 11. Method according to claim10, wherein the acidic solution comprises one or more anions selectedfrom the group consisting of nitrate and acetate.
 12. Method accordingto any one of the claims 10 or 11, wherein the alkaline solutioncomprises ammonium.
 13. Use of a catalyst according to any of the claims1-9 in a Fischer-Tropsch process or a functional group hydrogenation.14. Use of a catalyst according to any of the claims 1-9 in a slurryreactor, a loop reactor, a bubble column or a fluid bed reactor.